Voltage generating circuits are well known in the art and are used to provide a voltage output with defined characteristics. Known examples include circuits is adapted to provide a voltage reference, circuits having an output that is proportional to absolute temperature (PTAT) so as to increase with increasing temperature and circuits having an output that is complimentary to absolute temperature (CTAT) so as to decrease with increasing temperature. Those circuits that have an output that varies predictably with temperature are typically used as temperature sensors whereas those whose output is independent of temperature fluctuations are used as voltage reference circuits. It will be well known to those skilled in the art that a voltage generating circuit can be easily converted to a current generating circuit and therefore within the present specification for the ease of explanation the circuits will be described as voltage generating circuits.
One specific category of voltage reference circuit is that known as a bandgap circuit. A bandgap voltage reference circuit is based on addition of two voltages having equal and opposite temperature coefficient. The first voltage is a base-emitter voltage of a forward biased bipolar transistor. This voltage has a negative TC of about −2.2 mV/C and is usually denoted as a Complementary to Absolute Temperature or CTAT voltage. The second voltage which is Proportional to Absolute Temperature, or a PTAT voltage, is formed by amplifying the voltage difference (ΔVbe) of two forward biased base-emitter junctions of bipolar transistors operating at different current densities. These type of circuits are well known and further details of their operation is given in Chapter 4 of “Analysis and Design of Analog Integrated Circuits”, 4th Edition by Gray et al, the contents of which are incorporated herein by reference.
A classical configuration of such a voltage reference circuit is known as a “Brokaw Cell”, an example of which is shown in FIG. 1. First and second transistors Q1, Q2 have their respective collectors coupled to the non-inverting and inverting inputs of an amplifier A1. The bases of each transistor are commonly coupled, and this common node is coupled via a resistor, r5, to the output of the amplifier. This common node of the coupled bases and resistor r5 is coupled via another resistor, r6, to ground. The emitter of Q2 is coupled via a resistor, r1, to a common node with the emitter of transistor Q1. This common node is then coupled via a second resistor, r2, to ground. A feedback loop from the output node of A1 is provided via a resistor, r3, to the collector of Q2, and via a resistor r4 to the collector of Q1.
In FIG. 1, the transistor Q2 is provided with a larger emitter area relative to that of transistor Q1 and as such, the two bipolar transistors Q1 and Q2 operate at different current densities. Across resistor r1 a voltage, ΔVbe, is developed of the form:
                              Δ          ⁢                                          ⁢                      V            be                          =                                            K              ⁢                                                          ⁢              T                        q                    ⁢                      ln            ⁡                          (              n              )                                                          (        1        )            where    K is the Boltzmann constant,    q is the charge on the electron,    T is the operating temperature in Kelvin,    n is the collector current density ratio of the two bipolar transistors.
Usually the two resistors r3 and r4 are chosen to be of equal value and the collector current density ratio is given by the ratio of emitter area of Q2 to Q1. In order to reduce the reference voltage variation due to the process variation Q2 may be provided as an array of n transistors, each transistor being of the same area as Q1.
The voltage ΔVbe generates a current, I1, which is also a PTAT current. The voltage of the common base node of Q1 and Q2 will be:
                              V          b                =                              2            ⁢            Δ            ⁢                                                  ⁢                          V              be                        *                                          r                2                                            r                1                                              +                      V                          be              ⁢                                                          ⁢              1                                                          (        2        )            By properly scaling the resistor's ratio and the collector current density the voltage “Vb” is temperature insensitive to the first order, and apart from the curvature which is effected by the base-emitter voltage (Vbe) can be considered as remaining compensated. The voltage “Vb” is scaled to the amplifier's output as a reference voltage, Vref, by the ratio of r5 to re:
                              V          ref                =                                            (                                                2                  ⁢                  Δ                  ⁢                                                                          ⁢                                      V                    be                                    *                                                            r                      2                                                              r                      1                                                                      +                                  V                                      be                    ⁢                                                                                  ⁢                    1                                                              )                        ⁢                          (                              1                +                                                      r                    5                                                        r                    6                                                              )                                +                                    (                                                                    I                    b                                    ⁡                                      (                                          Q                      1                                        )                                                  +                                                      I                    b                                    ⁡                                      (                                          Q                      2                                        )                                                              )                        ⁢                          r              5                                                          (        3        )            Here Ib(Q1) and Ib(Q2) are the base currents of Q1 and Q2.
Although a “Brokaw Cell” is widely used, it still has some drawbacks. The second term in equation 3 represents the error due to the base currents. In order to reduce this error r5 has to be as low as possible. As r5 is reduced, the current extracted from supply voltage via reference voltage increases and this is a drawback. Another drawback is related to the fact that as the operating temperature of the cell changes, the collector-base voltage of the two transistors also changes. As a result of the Early effect (the effect on transistor operation of varying the effective base width due to the application of bias), the currents into the two transistors are affected. Further information on the Early effect may be found on page 15 of the aforementioned 4th Edition of the Analysis and Design of Analog Integrated Circuits, the content of which is incorporated herein by reference.
A very important feature of the Brokaw cell is its reduced sensitivity to the amplifier's offset and noise as the amplifier controls the collector currents of the two bipolar transistors.
An offset voltage, Voff, at the input of the amplifier A1 in FIG. 1 has a corresponding effect of imbalancing the currents I1 and I2 according to:I2r4−VOff=I1r3  (4)
The base-emitter voltage difference between Q1 and Q2, ΔVbe, reflected across r1 is:
                              Δ          ⁢                                          ⁢                      V            be                          =                                            K              ⁢                                                          ⁢              T                        q                    ⁢                      ln            ⁡                          (                              n                ⁢                                                                  ⁢                                                      I                    2                                                        I                    1                                                              )                                                          (        5        )            For r3=r4 we can get:
                              Δ          ⁢                                          ⁢                      V            be                          =                                                            K                ⁢                                                                  ⁢                T                            q                        ⁢                          ln              ⁡                              (                n                )                                              +                                                    K                ⁢                                                                  ⁢                T                            q                        ⁢                          ln              ⁡                              (                                  1                  +                                                                                    V                        off                                                                    Δ                        ⁢                                                                                                  ⁢                                                  V                          be                                                                                      ⁢                                                                  r                        1                                                                    r                        4                                                                                            )                                                                        (        6        )            The second term of (6) represents the error into the base-emitter voltage difference due to the offset voltage. This term can be reduced by making r4 larger compared to r1. However, by making r4 larger, the Early effect is exaggerated which is not desirable. A reasonable trade-off could be choosing the values of r4 and r1 such that r4=4r1. Using typical values for voltage reference circuits and assuming that r4=4r1, Voff=1 mV and ΔVbe=100 mV (at 25° C.) and the error due to the offset voltage in equation (6) is of the order of 0.065 mV. This error is reflected into the reference voltage according to equation (3). Assuming r2=3r1 and r5=r6 the offset voltage of 1 mV is reflected as 0.77 mV into the reference voltage. As the amplifier controls the collector currents each millivolt offset voltage is reflected as 0.77 mV error into the reference voltage. In the same way the amplifier's noise is reflected into the reference voltage, both of which are undesirable effects.
The “Brokaw Cell” also suffers, in the same way as all uncompensated reference voltages do, in that it is affected by “curvature” of base-emitter voltage. The base-emitter voltage of a bipolar transistor, used as a complimentary to absolute temperature (CTAT) voltage in bandgap voltage references, and as biased by a proportional to absolute temperature (PTAT) collector current is temperature related as equation 7 shows:
                                          V            be                    ⁡                      (            T            )                          =                                            V                              G                ⁢                                                                  ⁢                0                                      ⁡                          (                              1                -                                  T                                      T                    0                                                              )                                +                                    V                              be                ⁢                                                                  ⁢                0                                      ⁢                          T                              T                0                                              -                                    (                              σ                -                1                            )                        ⁢                                          k                ⁢                                                                  ⁢                T                            q                        ⁢                          ln              ⁡                              (                                  T                                      T                    0                                                  )                                                                        (        7        )            where:    Vbe(T) is the temperature dependence of the base-emitter voltage for the bipolar transistor at operating temperature,    VBE0 is the base-emitter voltage for the bipolar transistor at a reference temperature,    VG0 is the bandgap voltage or base-emitter voltage at 0K temperature,    T0 is the reference temperature,    σ is the saturation current temperature exponent (sometimes referred as XTI in computer added simulators).
The PTAT voltage developed across r2 in FIG. 1 only compensates for the first two terms in equation 7. The last term, which provides a “curvature” of the order of about 2.5 mV for the industrial temperature range (−40 C to 85 C) remains uncompensated and this is also gained into the reference voltage according to equation 3. An example of such curvature, which is a T log T effect, is given in FIG. 2.
As the “Brokaw Cell” is well balanced, it is not easy to compensate internally for the “curvature” error. One attempt to compensate this error is presented in U.S. Pat. No. 5,352,973 co-assigned to the assignee of the present invention, the disclosure of which is incorporated herein by way of reference. In this US patent, although the “curvature” error is compensated, in this methodology by use of a separate circuit which biases an extra bipolar transistor with constant current, it does require the use of an additional circuit.
Other known examples of bandgap reference circuits include those described in U.S. Pat. No. 4,399,398 assigned to the RCA Corporation which describes a voltage reference circuit with feedback which is adapted to control the current flowing between first and second output terminals in response to the reference potential departing from a predetermined value. The circuits serves to reduce the base current effect, but at the cost of high power. As a result, this circuit is only suited for relatively high current applications.
It will be appreciated therefore that although the circuitry described in FIG. 1 has very low offset and noise sensitivity, there is still a need to provide for further reduction in sensitivity to offset and noise.